Xenon is a peculiar element. It certainly has one of the most mysterious names (from the Greek xenos, or “stranger”). As a noble gas, it refuses to bond with other elements except under exotic conditions. And its uses are all about as creepy as its name: Folks use it for its eerie glow, to detect radiation, or in its liquid form to hunt for the universe’s dark matter.

But the element’s origin story is as mysterious as its name. There doesn’t seem to be an explanation for how our atmosphere’s xenon makeup came to look the way it does. After observing the element in the 67P/Churyumov Gerasimenko comet, scientists think some of it arrived on Earth from comets.

“The isotopic composition of atmospheric xenon requires a component not known until now,” Bernard Marty, the paper’s first author from the Ecole Nationale Supérieure de Géologie in France told Gizmodo. “We found this component. It’s cometary.”

Atoms of the same elements have different flavors, called isotopes, that differ solely in the number of neutrons in their nuclei. The isotope composition of our atmosphere’s xenon is a complex story, with lots of contributions from lots of places, such as the solar wind and meteors. After some corrections and analysis, scientists realised that there should have been a primordial xenon component in the early Earth’s atmosphere with a special isotopic trail mix. But the solar wind and meteor components don’t completely account for the primordial xenon’s makeup—together, they were missing some of the raisins and peanuts. They needed something to account for the extra bits that would have more of the lighter isotopes and fewer of the heavier ones.

“You have a special isotopic composition of atmospheric xenon that requires something not known up to now,” said Marty.

The researchers analysed the amount of xenon on the comet 67P/Churymov-Gerasimenko using instruments on board the Rosetta comet orbiter. After some analysis and summing, they realised that the comet had the xenon composition they were looking for—the missing xenon could have come from comets. They even came up with a number: 22 percent of our planet’s xenon, give or take five percentage points, could have come from comets. “This is a smoking gun for comet contribution to the Earth’s atmosphere,” said Marty. They published their results in the journal Science yesterday.

This does require an assumption that 67P/C-G is typical of the comets that would have brought xenon to the Earth, though. “This is based on measurements on this one comet, and the study of materials in the solar system always emphasises how diverse chemistry is throughout the solar system,” Colin Jackson at the Smithsonian Institution in Washington DC told Leah Crane at New Scientist (hi Leah!).

But if the conclusion is correct, it adds evidence to the fact that comets could have brought lots of stuff to the planet. That shouldn’t be surprising, though—you already knew you were just space dust. [Science via New Scientist]